1. Field of the Invention
The invention relates generally to suspension components on vehicles, and more particularly to a shock absorber with a damper valve that incorporates magnetic bias to improve valve performance. Such a device may be referred to as a threshold valve.
2. Description of the Related Art
Conventional shock absorbers employ a piston in a cylinder containing a substantially incompressible fluid. Orifices in the piston and passages leading to a fluid reservoir regulate the flow of oil so as to damp the oscillation of a suspension spring. In more advanced designs, the orifices include sprung valves, which commonly take the form of holes covered by flexible shims made of elastically deformable material, such as spring steel. Valves of this design open progressively with greater force and can be used to damp low-speed compression and/or extension of shock absorbers while preventing pressure “spikes” and consequent harshness of ride when the suspension must compress deeply and quickly. They can also permit the shock to extend rapidly after deep compression while slowing it sufficiently near maximum extension to prevent harsh “topping out”. Though superior to simple orifice dampers, dampers with flow-sensitive shims have limited potential for distinguishing between bumps and movements of the vehicle chassis.
In recent years some shock absorbers have been equipped with damper valves that are electromechanically actuated and are controlled by an electronic feedback system, sometimes in combination with a compressor for selectively varying fluid pressure in the damper. The nature of a given suspension event in such an apparatus is determined by computational projection, and then adjustments to damping resistance are made according to programmed instructions.
An approach that is less complex than electronic control yet more sophisticated than traditional shimmed orifices employs valves that are biased toward the closed position by the pressure of a confined gas or a preloaded mechanical spring. These devices provide relatively stiff damping resistance up to a certain threshold of applied force. Once the threshold is reached and the valve begins to open, relatively little additional force is required to move the valve to its fully open position, since a gas spring or preloaded mechanical spring can be designed to offer resistance along a gently sloped plot of load vs. deflection. This kind of valve makes possible relatively heavy damping of forces that are gradually applied to the suspension of a vehicle, such as rearward chassis movement during acceleration (“squat”), forward movement during braking (“dive”), and side-to-side tilt toward the outside of a curve (“roll”) while providing somewhat lighter damping of rapid, forceful movements of the vehicle wheel as it encounters bumps and depressions in the road surface.
Yet another approach is to employ what is commonly known as an inertia valve in a compression and/or rebound circuit of the shock damper. This type of valve consists of a weighted element (or elements) supported by or suspended from a mechanical spring (or springs). The element covers an oil port, acting as a blocker, and is of such a weight relative to the spring constant of the supporting spring that the element is dislodged, and the port consequently opened, only by upward or downward acceleration of the vehicle wheel.
The variable damping response afforded by preloaded shock valves and inertia valves as described above is particularly desirable for off-road bicycle suspension systems. In order to climb hills, the bicycle rider typically must stand on the pedals and pull vigorously on the handlebars, causing the rider's body weight to shift side-to-side and fore-aft. Conventional suspension damping allows unwanted “bobbing” of the bicycle and loss of pedaling efficiency when the rider's weight shifts in this way. The off-road bicycle application therefore places a premium on dampers that offer increased resistance to rider-induced suspension movement while minimally compromising sensitivity to road-induced suspension movement, such as bumps.
Although a preloaded, sprung damper valve may open along a gently sloping load vs. deflection plot once its force threshold is reached, its sensitivity nevertheless is limited by the continued upward direction of the gradient. In the case of rapid compression, when a bump is encountered the initial vertical acceleration or “shock force” causes a sudden rise in pressure that begins to open the preloaded valve. But this acceleration, with its attendant peak pressure, fades long before the wheel finishes surmounting the bump, thereby allowing the valve to close prematurely under the countervailing force of the spring. With the valve closed, damping resistance increases and a substantial part of the bump force is transmitted to the vehicle chassis. As this transmission of bump force increases, the ride becomes harsher and the vehicle's traction over irregular surfaces becomes poorer.
Similarly, in the case of rapid extension of a shock in which the rebound damping circuit is governed by a preloaded, sprung valve, the spring that urges the valve to close may overcome the force exerted by oil flow through the rebound circuit before the shock absorber fully extends, thereby reducing the available stroke of the suspension and adversely affecting ride quality.
The rebound performance objective during extension is rapid recovery from deep compression followed by smooth deceleration as extension is reached. Valves biased toward the closed position by mechanical springs necessarily limit the extent to which dampers can achieve this objective, because the spring force applied increases as the valve is opened further, thereby requiring an increasing force to maintain the valve in an open position at a time when the valve-opening force inherently decreases.
Inertia valves are subject to an analogous problem due to the progressively increasing resistance of a coil or leaf spring as it deflects. The sprung element that acts as a valve blocker, after having been dislodged by acceleration of the vehicle wheel as it moves over a bump, tends to return to the closed position before the bump has been fully negotiated. The result, again, is transmission of bump force to the chassis. Even in designs where the movement of the sprung blocker element is itself hydraulically damped, the spring return force is sufficient to impart inertia to the element. The inertia imparted by the spring to the blocker element may vary undesirably the response of the valve to accelerations of the vehicle wheel as it traverses bumps of different sizes at different frequencies.
In view of the above, the need exists for a damper valve that is biased toward the closed position at least partly by a force that does not increase as the valve opens.